Safe handling and disposal of radioactive material, such as nuclear waste, is an issue of great concern to society as a whole. Much attention has been focused on how to dispose of radioactive waste. Less attention has been focused on how to prevent radioactive contamination of porous surfaces of equipment used to handle radioactive material. Unfortunately, much of this handling equipment is made of anodized aluminum, which has a highly porous surface.
A substantially transparent "natural" oxide layer forms at the surface of aluminum upon exposure to air. The oxide layer prevents direct contact between the underlying aluminum and corrosive materials in the surrounding environment. Unfortunately, this "natural oxide" layer does not always have a uniform thickness. Because of this, natural oxides generally are removed from aluminum products, and the product then is "anodized," or controllably oxidized, to provide a protective oxide layer with better quality and substantially greater thickness.
Anodizing processes generally involve the use of a bath containing an electrolyte, such as sulfuric acid, oxalic acid, chromic acid, phosphoric acid, or combinations thereof, with or without certain addition agents. The aluminum workpiece generally is used as an anode and a component made of steel or other suitable material is used as a cathode. The anode and cathode are immersed in the electrolyte solution, and a direct or alternating current is passed through the electrolyte.
Although anodizing imparts satisfactory corrosion resistance to aluminum components, anodizing also suffers from several disadvantages. One disadvantage is the porosity of the resulting surface oxide. A typical anodizing treatment results in a porous polygonal cellular microstructure superimposed on a thin (less than 100 nm) "barrier" layer. The diameter of the pores in the microstructure can be as small as 10 nm. The cell dimension can be as small as about 30 nm.
The pores formed at the surface of anodized aluminum are undesirable because they tend to serve as corrosion sites which give rise to deep pits, and can result in "blooms" or white spots on the surface of the aluminum. Where the aluminum equipment handles radioactive material, the pores in the anodized aluminum surface can create a particularly acute problem. If the pores are not adequately sealed, then radioactive material can become trapped in the pores, rendering the equipment unsafe.
The pores of anodized aluminum customarily are sealed by immersion in a hot Solution containing hexavalent chromium. A solid compound of chromium, aluminum, oxygen, and some hydrogen forms within the pores. This solid compound seals the pores against penetration by corrosive agents. Unfortunately, the process does not purge the pores at the surface of the aluminum before or while the chromate sealant is formed. As a result, at least some gas remains in many of the pores, allowing the pores to serve as corrosion sites. Where the surface contacts radioactive material during use, these same sites may accumulate radioactive contamination. Hexavalent chromium solutions also are toxic, and their use and disposal creates additional environmental concerns.
The present invention provides an effective and non-toxic method for preventing radioactive contamination of porous surfaces--preferably anodized aluminum surfaces.